Thermostatic control for dry kilns



W. JACKSON THERMOSTATIG CONTROL FORDRY KILNS Filed May 2l, 1945 April 25, 1944.

.Invenoh 0.1622119122 Jackson mi C7. @a-M Patented Apr. 25, 1944 THERMOSTATIG CONTROL FOR DRY KHLNS.

William Jackson, Collingswood, N. J., assignorto B. F. SturtevantCompany, Boston, Mass.

Application May 21, 1943, Serial No. 487,895

6 Claims.

This invention'relates to the thermostatic control of the drying air in reversible flowr dry kilns and has as its object the maintaining ofthe desired temperatures and humidities in the drying air for the most effective drying of thematerial under treatment.

In drykilns for the drying of lumber, for example, it is customary for preventing warping and checking and case hardening, etc., during the drying, to dry the lumber in a number of different runs using drying air in the first run having a relatively high'relative humidity and relatively loW temperature, and decreasing the relative humidity of and increasing the temperature of, the drying air in the following runs as experience has shown to be desirable.

It has also been customary through the use of reversible fans or reversible flow dampers toperiodically reverse the flow of the air` through the kilns so thatthe lumber formerly in the leaving air stream is'in the entering air stream, etc'.

Itv has been diiicult however, to maintain the proper relative humiditiesin the drying air because the lumber itself gives off Vmoisture during the drying process and-the prior controls have not takenthis-into account particularly'in the -caseofi-reversible flow kilns.

This invention providesfor maintaining the correct humiditiesin thedryingairbyhaving a control instrument such as a; Wet` bulb thermostat `so located and arranged in a reversiblevflow dry kiln that it responds always to the condition of the air entering the lumber regardless of the Idirection of airv flow. Such a thermostat may .actuate simultaneously, dampers inthe -fresh air inlet and in the exhaust `air outlet, toadmit an increased volumeof fresh air and to lexhaust an increased volume of moist `air when the relative humidity rises above the desired level, and to admita decreased volume of fresh airand to exhaust a decreased volume ofmoist air Whenthe relative humidity falls below the desired control point.

After the Wet bulb control point `has been changed to a higher temperature. or if dueto some unusual condition or emergency .such as the kiln doors being openedthe wetbulb. tem- ,perature drops, .then the'. control will functionto rst close Vbothdampers after Whichsteanr `will .be 1 admitted automatically through a .steam `j et, to quickly restore the relative humidity to the control point. AUnder normal operating conditions the dampers .alone will maintainltheffwet bulb. control point byy thev use of. -a..modulating damper control motor. whichwill. conflnefvariations from the control point to a very limited degree.

A feature ofthe invention;` resides in the provision'of a control instrument which can beset for any desired wet bulb temperature and which acts to adjust the above mentioned-dampersto positions conforming to -the departure ofthe drying air entering the lumber, from the desired wet bulb temperature, so that the dampers are openedproportionately tothe rise in Wet bulb temperature thus quickly restoring the desired condition.

Another feature of this invention resides in the method of progressively-loweringthe-relative humidity-of the drying air entering the lumber, on a predetermined .schedule as the drying progresses,- and maintaining the Wet bulb temperature on the schedule Vby varying the volume Aof fresh air -into and exhaust air from, the dryer.

The invention Will now-be described `With reference tothe drawing which is aview partially in horizontal section of aportion ofi areversble flow dry kiln embodying this invention.

The lumber tobe dried -is stacked as usual inthe interior 5 of the kiln. The centrifugal fan 6 connects through the fan inlet chamber il with rpassage 1 and the Aoutlet ofthe fan ,conneets through the usual heating and humidify.- ing chamber 9, with the foutletpchanrber l0.

The axial, fresh air inlet .Il of the :fan con.- tainsthe damper l2for controlling the Volume of fresh make-up air drawn intothe kiln. `The exhaust, moist air outlet I3 is AVfor discharging a portion of the 4moist air recirculatedthrough the fan, and containsthedamper; I4gfor controlling the volume of exhaust `air,

The perforated outside ducts I5 and the-in,- side perforated vduct serve to supply air from the fan intov the interior 5 of the dryer or to draw air therefrom dependingY upon the position of the dampers Il. In the position indicated by the drawing the dampers I'l are placed forcausing the outside ducts I5 to actV as supply ducts, .and the inside lduct I5, to act as an exhaust ducte The dampers If!v `connect -thefan inlet passage 'I with the passage IS leading to the inner duct i6 or with the', passage I8 leading to the outside ducts 1 5 andfconnects the outlet chamber Il) with the passages I8`to the` ducts ,l5-or; with the passage IiiA tol-the `duct l5. Infthedravvingthfe heavy linesillustratethe dempers l1 positioned to openthe? passages I-El-a`nd-v 2 l'lfrom the ductv i6 tothe-inlet passage 'l lwith and to close thepassages connecting the duct llB with the-fan outlet chamber I so that the air is supplied through the ducts I and is withdrawn for recirculation through the duct I6.

With the dampers I1 adjusted through angles of 90 to the positions shown by the dash-dot lines of the drawing, the outlet chamber I0 would be connected through the passage I9 to the duct I8, and the ducts I5 would be connected through the passages I8 and 20 with the fan inlet passage 1 so that Athe outside ducts would act as recirculation ducts and the duct I 6 would act as supply duct.

The dampers are periodically changed so that the air is withdrawn from those portions of the lumber formerly in the entering air and is supplied to the opposite portions from which formerly the air was withdrawn so'tha't all portions of the lumber are exposed to the same conditions for preventing portions drying more quickly or thoroughly than other portions.

The` dampers I2 and I 4 are simultaneously adjusted by the motor 2I through the linkage .elements 22, 23, 24,7125 and 26 so that the dampers are simultaneously moved throughout their entire ranges of from fully open to fully closed positions. The motor 2| may be of any Well known air or electric type and is actuated 'by the wet bulb thermostat 21 in the kiln chamber 5. e i' The thermostat 21 is placed in an enlargementl 28 in the relatively small duct 29. The duct 29 opens at one end into the inside duct I6 and at the other end into one of the outside ducts I5. With this arrangement regardless of the direction of ow of the drying air, the control ther- Vmostat is exposed to the drying air entering the lumber. The arrows on the drawing illustrate vthe air flow fromthe outside ducts I5 acting as supply ducts, through the kiln and out the inside duct I6 acting as the recirculation duct. The `aircontacting theY thermostat 21 has the condition of that supplied through the outside ducts I5.

When the dampers are adjusted for reverse flow. the supply air will flow through the inside duct Iv and the outside ducts I5 Will act as recirculated air ducts. When this happens, a .portion of the air from the duct I wil1 pass in contact with the thermostat 21. So regardless Vof the direction of air flow the thermostat responds to the condition of the drying air entering the material to be dried.

The thermostat 21 is set for the desired control wet bulb temperature and adjusts the dampers I2 and I4 automatically to maintain that temperature. The following schedule may be followed in the drying, for example, of certain hardwood lumber.

If the wet bulb temperature during a run increases above the desired temperature, the thermostat 21 adjusts the dampers I2 and I4 towards open position until the wet bulb temperature is reduced to the desired point, the degree of movement of the dampers being proportional dto; the degree of, departure of the ,Wetbulb tem.-

perture from the desired point. Likewise if the wet bulb temperature falls below the desired point, the dampers I2 and I4 are adjusted towards closed position.

By way of example, if the Wet bulb temperature falls, say 2 below the control temperature, the thermostat 21 acts to cause the motor 2| completely to close the dampers.

When the thermostat acts completely to close the dampers it also adjusts the valve 28 in the supply to the steam humidifier for increasing the humidity of the air in the dry kiln. Upon rises in wet bulb temperature the reverse action takes place.

The thermostat 21 and the motor 2I preferably are sensitive enough that the dampers may be fully closed and the humidifier rendered effective at temperature drops of as low as 1/2". The variation between 1/2 and 2 will allow the operator to vary the sensitivity depending upon the kind and thickness of the lumber being dried.

Since the control thermostat responds to the condition of the air entering the lumber regardless of where it enters same, the supply air iirst entering the lumber is at the correct temperature and relative humidity for proper drying. Prior controls resulted in too high temperatures and too low relative humidities in the air entering the lumber.

The control system of this invention has been found particularly elective in the drying of thick hard woods which are green from the saw or are comparatively green from the saw.

The dry bulb temperatures of the above schedule are maintained by the usual dry bulb thermostat which also may be placed in the duct 29 and by associated apparatus which are not illustrated.

.As illustrated the fresh air inlet duct II is larger than the exhaust duct I3 so that throughout the entire range of adjustments of the dampers, a substantial static pressure is maintained in the kiln assuring a positive discharge pressure in the exhaust duct I3.

While one embodiment of the invention has been disclosed for the purpose of illustration, it should be understood that the invention is not limited to the exact apparatus and arrangement of apparatus illustrated, as modifications thereof may be suggested by those skilled in the art, without departure from the essence of the invention.

What is claimed is:

1. A dry kiln comprising a drying chamber, ducts connecting opposite sides of said chamber, one for supplying drying air into, and the other for withdrawing drying air from, said chamber, means for reversing the iiow of air through said ducts, means for changing the condition of the air supplied into said chamber, means responsive to said condition of the drying air for adjusting said last mentioned means, and a sampling duct interconnecting said ducts and by-passing said chamber and enclosing said responsive means for supplying samples of the air entering said chamber in contact with said responsive means.

2. A dry kiln comprising a drying chamber, ducts connecting opposite sides of said chamber, one for supplying drying air into, and the other for withdrawing drying air from, said chamber, means for reversingthe flow of air through said ducts, means for changing the humidity of the air supplied into said chamber, means responsive to the humidity of the drying air for adjusting said last mentioned means, andi a sampling duct interconnecting said ducts and by-passing said chamber and enclosing said responsive means for supplying samples of the air entering said chamber in contact with said responsive means.

3. A dry kiln comprising a drying chamber, ducts connecting opposite sides of said chamber, one for supplying drying air into, and the other for withdrawing drying air from, said chamber, means for reversing the ow of air through said ducts, means for changing the humidity of the air supplied into said chamber, a Wet bulb thermostat for adjusting said last mentioned means, and a sampling duct interconnecting said ducts and by-passing said chamber and enclosing said thermostat for supplying samples of the air entering said chamber in contact with said thermostat.

4. A dry kiln comprising a drying chamber, ducts connecting opposite sides of said chamber, one for supplying drying air into, and the other for withdrawing drying air from, said chamber, means .for reversing the flow of air through said ducts, means including fresh and exhaust air passages and dampers in said passages for changing the humidity of the air supplied into said chamber, means responsive to the humidity of the drying air for adjusting said dampers for increasing the volume of fresh air and decreasing the volume of exhaust air upon a rise in humidity, and a sampling duct interconnecting said ducts and ley-passing said chamber and enclosing said responsive means for supplying samples of the air entering said chamber in contact with said responsive means.

5. In a dry kiln having a fan, a drying chamber, oppositely placed recirculated air passages connected to said fan and chamber for recirculating drying air therethrough, a fresh air duct connected to the fan inlet, an exhaust air duct connected at the fan outlet, dampers in said ducts, and means for reversing the flow of drying air through said passages, the combination of a sampling duct interconnecting said passage 3, and means including means in said sampling duct responsive to changes in the humidity of the air passing therethrough for adjusting said dampers for increasing the volume of exhaust air and for decreasing the volume of fresh air upon a decrease in the relative humidity of the air sample.

6. In a dry kiln having a fan, a drying charnber, oppositely placed recirculated air passages connected to said fan and chamber ior recirculating drying air therethrough, a fresh air duct connected to the fan inlet, an exhaust air duct ccnnected at the fan outlet, dampers in said ducts, and means for reversing the flow of drying air through said passages, the combination of a sampling duct interconnecting said passages, and means including a Wet bulb thermostat in said sampling duct for adjusting said dampers for increasing the volume of exhaust air and for increasing the volume of outdoor air upon an increase in the Wet bulb temperature of the air sample.

WILLIAM JACKSON. 

